Many thing can be involve.

It sounds like death wobble to me.

My experence with D/W happens around the speed of 30-50  mph.

Question:
Do you have an after market lift, high steer, ect?

You want to check on the spring bushing but first I'll pull out an tape measure and measure the toe of your  front tires.  Too much in and too much out will cause it also.  Next check your tie rod ends it can be roasted.